1. Field of Endeavor
The present invention relates to the field of turbo-machines, such as, for example, wind turbines, steam turbines, gas turbines, and compressors. It relates to a method for influencing, in particular damping or suppressing, mechanical vibrations occurring during operation in a turbomachine blade. It also relates to a turbomachine blade useful for carrying out the method and to a piezoelectric damping element to be installed in such a turbomachine blade.
2. Brief Description of the Related Art
When turbines (or compressors) are in operation, asynchronous and synchronous blade vibrations may be generated due to aerodynamic effects (for example, fluttering) or to mechanical effects (for example, on account of the friction of the blades against the casing). Resonances in the blade may lead to problems of vibratory crack formation (“high-cycle fatigue” HCF) which constitutes a critical type of failure for turbine and compressor blades.
In order to protect turbomachine blades against such HCF faults, the blade leaves are coupled to integral shroud elements or winglets which increase the rigidity of the blade arrangement of a turbine stage and damp or suppress vibrations due to friction between the adjacent blades. Arrangements of this type are known, for example, from the publication EP 0 214 393 A1 or U.S. Pat. No. 3,752,599.
If the turbine concept requires free-standing blades, friction-generating devices may be arranged on the blades beneath the platforms or between the blades as friction pins or may be accommodated inside the blades. Such solutions are known, for example, from the publication EP 1 538 304 A2 or U.S. Pat. Nos. 4,460,314 and 6,979,180 B2.
The damping effect of such friction-generating devices and frictional couplings depends, however, upon the optimal normal force and rigidity of the coupling contact which have to be coordinated suitably with the relevant resonant frequency (to be damped). In other words, frictional damping can be used effectively only for a specific vibratory frequency of the blade, whereas other frequencies are damped inadequately or not at all.
Frictional dampers are highly dependent upon amplitude, and any variation in rigidity or in the mass system involved on account of abrasion or other modifying processes in the system results in changed resonant frequencies which are detrimental to the effectiveness of frictional dampers.
However, it has already been proposed (see, for example, the publication EP 0 727 564 A1), to damp vibrations of turbine blades in that, as a result of the interaction of permanent magnets with the blades, eddy currents are generated which are converted into heat loss. The range of use of such solutions is narrowly limited, however, because interaction is restricted to the region between the blade tip and the opposite casing wall. Vibrations occurring inside the blade leaf therefore cannot be effectively damped.
Furthermore, it is known (see, for example, the publication JP 2003138904), for actively controlling frictional coupling between adjacent turbine blades, to insert in the blades piezoelectric elements by which frictional contact during operation can be optimized and readjusted. This solution is not suitable for free-standing blades. However, piezoelectric elements of this type may also be used in order to measure and monitor the contact pressure of such frictional couplings (JP 2003138910).
Overall, the friction-based damping systems are complicated in terms of setup and use and can be frequency-tuned only with difficulty, while the principle based on the generation of eddy currents can be used to only a very limited extent.